Particle analysis is important in a wide variety of industrial processes including the fabrication of high performance semiconductor and optoelectronic devices. As feature sizes on semiconductor devices shrink, the size of particles that causes defects also decreases. On today's advanced devices, particles as small as 0.1 micrometers can cause yield reducing defects. Such particles can come from several sources including delaminating films, broken wafers, atmospheric dust, and the vacuum processes used for the deposition and etching of thin films, especially plasma processes. Analysis of the chemical composition of the particles is an important step in finding the root cause of particle contamination.
There now exist highly accurate techniques for detecting and analyzing sub-micron particles. Such techniques are described in U.S. Pat. No. 5,382,794 issued to S. W. Downey et al. on Jan. 17, 1995 and U.S. Pat. No. 5,631,462 issued to the present inventor W. D. Reents, Jr. on May 20, 1997, both of which are incorporated herein by reference. In essence, the particles are entrained within a gas stream, fragmented and ionized by a laser beam ("laser ablation"), and the chemical nature and concentration of the species within the particle is determined by mass spectrometry. This approach permits real time measurement of particles as small as 1 nm in diameter.
One difficulty impeding wider use of systems for detecting and analyzing such small particles is the need for calibrating them in situ. The equipment is delicate and must be calibrated on the site of use. Typically calibration is accomplished through the use of particle generating systems for producing particles of known size distribution. Such generating systems, however, are expensive specialty devices which are not easily transported. They typically must be disassembled for transportation and reassembled on site, a process which takes several hours at each stage. Alternatively pre-formed particle aggregates can be used in calibration, but specialized equipment is required for de-agglomeration, and the useable sizes are typically in excess of 20 nm. Accordingly there is a need for an improved calibration sample for particle analyzers.